1. Field
The invention is related to an all-dielectric, self-supporting (ADSS) fiber optic cable that mitigates the deterioration of the outer protective jacket by the dry-band arc or tracking mechanism. More particularly, the cable has a semi-conductive material that provides tracking resistance for the cable in the outer jacket.
2. Related Art
All-dielectric, self-supporting (ADSS) fiber optic cables have been used on power utility, high voltage rights-of-way for many years. The dielectric nature of the cable is ideal for safe installation on power transmission facilities above 69 kV line voltages. By their nature, the dielectric properties of the ADSS cables pose problems with a degradation of the outer protective jacket due surface arcs known as “Tracking” or “Dry Band Arcing.” This is caused by the capacitive charge from the power phase conductors to the ADSS cable and changes to the dielectric properties of the outer protective jacket due to surface contaminates and wetting/drying cycles of the cable surface during participation. When the cable is contaminated and has non-uniform resistivity due to wet and dry areas a “scintillation” or “arc” creates a thermal and mechanical erosion of the jacket material that potentially can damage the jacket material, expose the cable's strength members to degradation, and cause subsequent failure of the cable's ability to support the cable's mass and any additional wind and ice loads.
Existing technology to mitigate the deterioration of the outer protective jacket by the dry-band arc or tracking mechanism was directed to computer modeling of the electrical potential coupled to the ADSS from the phase conductors and setting maximum thresholds for different jacket materials based on the environment and different jacket materials with higher resistance to the deterioration caused by tracking. The special jackets would have properties of lower dielectric properties and be cross-linked with proprietary compounds that provided a higher resistance to thermal and mechanical stresses caused by the tracking mechanism. These proprietary jackets are very expensive and difficult to manufacturer. Many papers have been published since the mid 1980's describing the electrical stress mechanisms and the requirements of different jacket materials. See for example:    S. M. Rowland, Prevention of Dry-Band Arc Damage on ADSS Cables, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 13, No. 4; August 2006, pp. 765-772;    R. Sarathi, S. Chandrasekar, V. Sabari Giri, C. Venkataseshaiah and R. Velmurugan, Analysis of surface degradation of high density polyethylene (HDPE) insulation material due to tracking, Bull. Mater. Sci., Vol. 27, No. 3, June 2004, pp. 251-262;    Qi Huang, George G. Karady, Baozhuang Shi, and Monty Tuominen, Study on Development of Dry Band on ADSS Fiber Optic Cable, IEEE Transactions on Dielectrics and Electrical Insulation Vol. 12, No. 3; June 2005, pp. 487-495;    Qi Huang, George G. Karady, Baozhuang Shi, and Monty Tuominen, Numerical Simulation of Dry-band Arcing on the Surface of ADSS Fiber-optic Cable, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 12, No. 3; June 2005, pp. 496-503;    Cristian Militant, ADSS Cables Electrical Corrosion Tests, International Wire & Cable Symposium Proceedings 1999, pp. 614-620;    Optical Fiber Cables Near High Voltage Circuits, Corning Cable Systems Applications Engineering Note, AEN 32, Revision #5, 2002;    George G. Karady, Srinivasan Devarajan, Monty Tuominen, B. Han, Computer Simulation of Fiber-optic Cable Failures due to Dry-Band Arcing, 0-7803-5569-5/99 1999 IEEE, pp. 890-894; and    George G. Karady, Johnny Madrid, Assessing Deterioration of ADSS Fiber Optic Cables Due to Corona Discharge, PSERC Publication 02-17, May 2002.
Additional background is provided in FIGS. 1-6.
FIG. 1 shows induced voltage distribution on clean dry all-dielectric self-supporting fiber optic cable 1. This is caused by the capacitive coupling of the power from the phase conductors 2 to the ADSS fiber optic cable 1. This charge is driven to near zero as the ADSS cable 2 approaches the tower structure 3 and grounded cable attachment hardware 4 on either side of the span. This is the fundamental mechanism that leads to the cable tracking mechanism.
FIG. 2 shows the induced voltage potential V (in kV) and the electric field E (in kV/cm) on the ADSS cable 1 near the tower structure 3. The potential and field are inversely proportional, as the voltage potential drops the electric field increases. The graph shows the values from the tower structure 3 (or earthed deadend) to the middle of a span between two tower structures.
FIG. 3 shows the voltage potential on the ADSS cable as the cable goes through the wet and dry cycles of precipitation. This diagram show an ADSS cable that is clean and uncontaminated, thus the cable surface resistance is uniform over the majority of the span. For example, the top line represents a cable that is dry. It could have a cable resistance of approximately 1014 Ω/m. The bottom two lines represent cables that begin to get wet, and become more wet. They show the cable resistances dropping to approximately 1011 Ω/m and then to 109 Ω/m and then 107 Ω/m. As conditions change, the cable resistances can cycle from higher resistances to lower resistances, and from lower resistances to higher resistances.
FIGS. 4A-4D show the voltage distribution on an ADSS fiber optic cable as the cable develops a localized area of contamination or non-uniform drying. The show the distribution of a cable from the tower structure to about 100 m away from the support structure. A dry band is formed and the resistance on the ADSS cable jacket is high. With current flowing over the jacket this dry band area will have a short period where the current flowing on the surface will scintillate or arc across the dry band or contaminates. This arc causes degradation to the jacket surface that eventually caused exposure of the cable strength elements and subsequent cable failure. FIG. 4A shows the voltage distribution for a dry cable, such as the top line in FIG. 3. FIG. 4B shows the voltage distribution for a completely wet cable. FIG. 4C shows the voltage distribution of a wet cable, such as in the bottom line of FIG. 3, with a cable resistance of approximately 107 Ω/m. The cable also has a dry (or contaminated spot) approximately 30 meters from the tower structure with a cable resistance of approximately 109 Ω/m. FIG. 4D shows that over time, the voltage potential at the dry spot can get worse, further damaging the cable. For example, as shown, the cable resistance at the 30 meter spot has increased to approximately 1012 Ω/m.
FIGS. 5A-5C show how the tracking etch starts the erosion process to damage the cable jacket. FIG. 5A shows moisture droplets on an ADSS cable. In this example, the cable is very wet there isn't an issue because the charge can continue to flow between the moisture droplets. However, in FIG. 5B, if the cable semi-wet, a tracking etch, or arc can occur if the droplets are further apart. This is shown by the lines on the cable. FIG. 5C shows that tracking etch lines that remain on cable after it has dried. This can create a preferred path for the charges that are built up on the cable.
FIG. 6 shows an ADSS cable that has started to experience the jacket erosion caused by the tracking mechanism.
An object of this invention is to develop cable that mitigates the deterioration of the outer protective jacket by the dry-band arc or tracking mechanism.
An object of this invention is to develop cable that mitigates the deterioration of the outer protective jacket by the dry-band arc or tracking mechanism that is less expensive to manufacture than cables with special jackets with proprietary compounds that provided a higher resistance to thermal and mechanical stresses caused by the tracking mechanism.